The present invention relates to processes for producing yarn, to yarns produced by the processes, to fabrics comprising the yarn and to transmission belts comprising such fabrics.
Existing transmission or timing belts, when used in engines or the like, are designed with teeth which mesh with appropriate pulleys to provide synchronised power transmission. The transmission belts are formed by pressing a suitable resilient fabric over a mould to mould the teeth and impregnating the fabric with suitable elastomer. Distortion of the teeth during use leads to wear on the belt and is a mode of failure. Additionally, the distortion of the teeth leads to elastic hysteresis, which in turn leads to the build up of heat.
Nylon has usually been chosen as a fabric forming the surface of the belt which contacts the pulleys. Nylon has the advantage of resilience when textured, with good heat and wear resistance, under the temperatures and under the conditions in current engines. However, there is a need for yarns that can be used to form fabrics for transmission belts that are able to withstand higher temperatures and also with enhanced chemical stability because engines in the future are likely to run at higher temperatures to improve efficiency and may also have a more aggressive chemical environment. There is, in any case, a need for transmission belts that do not require replacement during the lifetime of an engine. Yarns contributing to these advantages would also have uses in many other technical fabrics.
There have been attempts to increase the heat or wear resistance of fabrics by incorporating other materials such as PTFE. WO-A-2003/031700 relates to a fabric that can be used in a timing or transmission belt that contains PTFE to reduce friction. In WO-A-2003/031700, a yarn is disclosed having an elastic core thread which is surrounded by a thread comprising a friction reducing substance, e.g. PTFE.
There have also been attempts to use heat and chemical wear resistant polymers such as aramid filament in yarns. Unfortunately, aramid filaments alone have a very high modulus and so are not elastic enough to enable good forming on the mould or be acceptable in use as a transmission belt. A combination of aramid with nylon and elastomer has been attempted, but can introduce further problems because elastomers can degrade leaving voids in the transmission belt that possibly accelerate wear and reduce life. Use of texturised nylon and aramid filament leads to problems on activation (using methods such as scouring) because, after the yarn bulks, the aramid filament is forced out of the bundle causing loops. Loops cause disadvantages including reduced abrasion resistance and hence reduced durability, reduced consistency of the final product, reduction of the dynamic performance of the fabric especially in terms of stretch characteristics and a reduced or inconsistent adhesion performance.